Tau immunotherapy for Alzheimer's disease and related tauopathies has advanced from proof-of- concept studies (Sigurdsson, EM, NIH R01AG020197, 2001; Asuni AA et al, J Neurosci, 27, 2007) to clinical trials. Because tau pathology correlates better with the degree of dementia than amyloid-? (A?) pathology, it is likely that targeting tau will be more effective than clearing A? in the later stages of the disease, when cognitive impairments are evident. Even though a few clinical trials have been initiated after confirmation and extension of the original studies, we have a very limited understanding of the mechanisms involved in antibody-mediated clearance of tau pathology. Some insight has been obtained on which epitopes to target and the mechanism of action but much remains to be clarified. Interestingly, tau antibodies interact with the protein both extra- and intracellularly but the importance of each site for tau clearance is not well defined, and is likely antibody-dependent. Some antibodies are readily taken up into neurons whereas others are not and these differences are likely charge- dependent. As most of tau is found intraneuronally, targeting it there is likely to be more efficacious than only outside neurons. The overall hypothesis of the project is that tau antibodies can slow the progression of tau pathology in humans. The Specific Aims are: 1) To determine how charge, isotype, affinity and size influence antibody efficacy and to clarify the mechanisms involved, and; 2) Antibody engineering based on structural characterization to improve efficacy and for humanization.Towards these aims, we have established collaborations with investigators at our university, who have complementary interests and expertise. The project will focus on antibodies against a key tau epitope, phospho-serine 396,404, which we targeted in our pioneering studies, and it has now been confirmed to be a feasible target by several groups. We will study how charge, isotype, affinity and size influence the efficacy. Furthermore, we will examine how antibody uptake and glial cells influence efficacy, assess proteomic changes associated with antibody- mediated clearance of tau pathology to identify the pathways involved, and engineer the antibodies to clarify mechanisms and improve efficacy. This program should advance our mechanistic understanding of tau immunotherapies and how those can be improved to treat Alzheimer's disease and related tauopathies, with direct relevance to various other protein misfolding disorders.